Geospatial HSPF model of the Sandies and Elm watershed, Texas

The Sandies and Elm Creeks were placed on EPA’s 303(d) list in 2000 due to depressed dissolved oxygen and elevated bacteria levels. Given the rural, agricultural nature of the watershed, a Total Mass Daily Load (TMDL) study was initiated to determine the source or sources of the non-point source pollution. A model needed to be developed that simulated the agricultural runoff from the watershed. The simulation model, Hydrologic Simulation Program – FORTRAN (HSPF) was chosen. A typical HSPF model was conceived, but during the course of the study circumstances forced the model to develop in an atypical way. The classic source of precipitation forcing data, the National Climatic Data Center, lacked point precipitation stations with data during the calibration timeframe; therefore alternate data sources were reviewed and NEXRAD data was chosen as the alternate data source. But, the use of NEXRAD data required that the model be distributed to a greater degree than a classic HSPF model. This delineation pushed the HSPF code to the edge of its design and encouraged examination of the weaknesses of both HSPF and hydrologic modeling in general.